Atmospheric diffuser improved lower screens backflushing

ABSTRACT

In an atmospheric diffuser for the treatment of paper pulp (thickening, bleaching, and/or washing) the flow of backflushing liquid is controlled so as to maximize the effective treatment of the pulp. The pulp passes upwardly in a vessel. A number of annular screen assemblies are provided, with at least one screen associated with each screen assembly. The lower screen includes a barrier wall with a passage in the barrier wall adjacent the bottom end of the screen. Liquid which flows through the screen must flow to the passage in order to be withdrawn from the vessel, as the screen is slowly moved upwardly. When the screen is rapidly moved downwardly to cause backflushing, the backflushing liquid at the top end of the screen is liquid that recently passed through the screen at that point, so the pulp at that point--which is the cleanest along the length of the screen--has a minimal chance of being contaminated. The barrier may be provided by a number of tubes separated by webs, or a number of channels separated by webs. A baffle may be mounted between each passage and screen to redirect the backflushing liquid so that it does not merely backflush the bottom portion of the screen.

BACKGROUND AND SUMMARY OF THE INVENTION

An atmospheric diffuser is a successful piece of equipment for treatingpaper pulp and the like. Although such structures can be used only forthickening, typically diffusers are used for washing, bleaching, orotherwise treating pulp, with a treatment liquid. The diffusers areoften provided with multiple stages, having a number of screenassemblies. Typical multiple stage diffusers are illustrated in U.S.Pat. Nos. 3,815,386 and 4,172,037, the disclosures of which areincorporated by reference herein.

In conventional atmospheric diffusers, normally the pulp flows upwardlyin the diffuser vessel, flowing past the bottom and then past the top ofscreens extending downwardly from an arm which receives liquid whichpasses through the screens, for ultimate withdrawal from the vessel. Intests on the efficiency of diffusers, it was surprisingly found that thelower screens in each stage--the screens that extend downwardly from themounting arm, when the pulp is flowing upwardly in the vessel --hadsignificantly less efficiency than the upper screens of each stage.After investigation and analysis, it was determined that the source ofthis problem of relatively poor screen efficiency was contamination ofthe cleanest portion of the pulp as it flowed upwardly with backflushliquid that was dirtier than that just withdrawn from the pulp at theupper end of the screen.

In order to solve the above identified problem, according to the presentinvention, the pathway that the liquid being withdrawn from the pulpmust take as it passes through the screens, is down toward the bottom ofthe screen. Thus, when backflushing action occurs by rapid downwardmovement of the screen, the backflushing liquid at the top of the screenwill be relatively clean liquid which has just passed through the screenat that point, rather than much dirtier liquid in conventionalatmospheric diffusers. The pulp is preferably caused to take this pathby providing barriers--such as tubes or channels--within the screenswith passages adjacent the second ends of the screens. In order to makethe backflushing action as uniform as possible, a baffle is mounted infront of the passage--between the passage and the screen--to redirectthe backflushing liquid flowing through the passage.

According to one aspect of the present invention, an atmosphericdiffuser for treating cellulosic fibrous material pulp is provided. Thediffuser comprises: a generally upright vessel; a plurality of armsmounted within the vessel for up and down movement; a plurality ofannular screen assemblies mounted on the arms, each assembly includingat least one screen; conduit mean for transporting liquid passing intothe screen assemblies from pulp in the vessel, into the arms and out ofthe vessel; at least one screen of each screen assembly having an outerface and an inner face, the outer face first encountering the pulp as itmoves in its direction of movement, with the arms operatively connectedto the screen face at the downstream end thereof in the direction ofpulp movement; means for moving the arms up and down in movementcoincident with pulp flow in one direction and to cause backflushing ofliquid through the screens in the other direction; and means fordirecting backflushing liquid from the inner face of the screen backthrough the screen so that the backflushed liquid passing through thescreen adjacent the downstream end thereof has substantially the sameproperties as liquid just withdrawn through the screen at that point.The means for directing the backflushing liquid preferably comprisebarrier means with passages in the barrier means adjacent the second endof the screens. Baffle means are mounted between the passages and thescreen innerface for redirecting the backflushing fluid.

The barrier means preferably comprise a plurality of tubes and areconnected by webs, or a plurality of channels that connected by webs.Preferably, each screen assembly includes two concentric screens withthe barriers disposed within the annular space between the screens, inwhich case the barrier tubes or channels may each have two passagestherein--one facing each screen--or alternate tubes or channels can havea passage facing one way, with the next tube or channel passage facingthe other way.

According to another aspect of the present invention, a method oftreating cellulosic fibrous material pulp in an upright vessel isprovided. The method comprises the steps of: (a) causing the pulp toflow in a first vertical direction in the vessel, the pulp when flowingin the first direction initially encountering the second end of thescreen, and the flowing to and past the first end thereof, some liquidin the pulp passing through the screen; (b) withdrawing liquid flowingthrough the screens from the vessel; (c) periodically causing a flow ofbackflushing liquid back through the screen into the pulp; and (d)controlling the backflushing liquid so that the liquid backflushing thescreen at the first end thereof is liquid that passed through the screensubstantially at the first end of the screen. The method also preferablycomprises the step (e) of introducing treatment liquid into the pulpadjacent the annular screen. Step (d) is practiced by defining a pathrequiring flow of pulp passing through the first end of the screen tothe second end of the screen. The first vertical direction--thedirection of flow--of the pulp preferably is upwardly, and the screen ismoved slowly upwardly with the pulp when treatment liquid is passingthrough the screen, and is moved quickly downwardly to effectbackflushing.

According to another aspect of the present invention, a screen assemblyper se is provided. The screen assembly comprises: first and secondannular bars; at least one annular screen; the screen connected at afirst end thereof to the first bar, and at the second end thereof to thesecond bar; means defining a plurality of spaced through extendingopenings in the first bar; a barrier extending from the first barsubstantially to the second bar and spaced from the screen, the barrierseparating the screen from the plurality of openings; and means defininga plurality of passages in the barrier adjacent the second end of thescreen so that liquid passing through the screen must flow through thepassage to enter the openings. Preferably two concentric screens areprovided with the barrier between them, and the barrier comprises aplurality of tubes separated by webs, or a plurality of channelsseparated by webs.

It is a primary object of the present invention to provide for the moreefficient treatment of pulp or the like in atmospheric diffusers or thelike. This and other objects of the invention will become clear from aninspection of a detailed description of the invention, and from theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the lower screen of a typical priorart screen assembly for an atmospheric diffuser;

FIG. 2 is a view like that of FIG. 1 only showing an exemplary lowerscreen of the screen assembly according to the invention;

FIG. 3 is a cross-sectional view partly in elevation of an exemplarypulp-treating assembly according to the invention, utilizing lowerscreens as illustrated in FIG. 2;

FIG. 4 is a top view, partly in cross section and partly in elevation,of an arc of a lower screen assembly according to the invention;

FIG. 5 is a longitudinal cross-sectional view at the passage and baffleplate of one embodiment of the screen assembly according to the presentinvention; and

FIG. 6 is a view like that of FIG. 5 only for a second embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

Generally indicated by reference numeral 1 in FIG. 1 is a conventionallower screen assembly of a conventional atmospheric diffuser, such asshown in U.S. Pat. Nos. 3,815,386 or 4,172,037. This screen assemblyincludes a lower annular bar 2, an upper annular bar 3, a connecting web4 between the bars 2, 3, first and second concentric annular screens 5,6 extending between the bars 2, 3, and a header 7 (ultimately attachedto a supporting arm) for receiving liquid which passes through theopenings 8 in the bar 3 to be withdrawn from the diffuser. Asillustrated on the left hand side of FIG. 1, typically the pulp movesupwardly, as the screen assembly 1 moves upwardly at substantially thesame speed as the pulp movement, spent treatment liquid (e.g. wash wateror bleach liquid) passes through the screen 5 and ultimately into thepassage 8, to be discharged from the diffuser. Once the screen assembly1 reaches its upper limit of travel, the screens 5 and 6 arebackflushed--to prevent matting of the pulp thereon, which would clogthe screen openings--by rapidly moving the assembly 1 downwardly. Thisis illustrated schematically on the right hand side of FIG. 1.

When the screen assembly 1 is reciprocated downwardly rapidly, theliquid between the screen 6 and web 4, and even some of the liquid inthe passages 8 and possibly even header 7, flows outwardly through thescreen as indicated by the horizontal arrows in FIG. 1. The problem isthat the backflushing liquid at the top of the screen--adjacent bar3--is a mixture of the dirtiest liquid, which passes through the bottomend of the screen (closest to the bar 2) upwardly toward the openings 8,as well as the liquid which has just passed through the top of thescreen. Since the pulp gets progressively cleaner as it moves upwardly,as indicated by the liquid passing through the screens 5, 6 beingprogressively cleaner at progressive points along the screen, when thebackflushing action occurs, the liquid in the zone 9 in FIG. 1 isdirtier than the rest of the liquid in the pulp at that zone, causing aslug of contaminated pulp at that zone, and thereby reducing theefficiency of the screen assembly 1.

The screen assembly illustrated generally by reference numeral 10 inFIG. 2 solves the problem of contaminating relatively clean pulp withrelatively dirty liquid which is illustrated in FIG. 1. The screenassembly 10 includes annular mounting bars 11, 12 at the second andfirst end of the annular concentric screens 14, 15, respectively, theupper bar 12 connected to a header 13 which is ultimately connected to asupporting arm. Barrier means 16 are provided interiorly of the screens14, 15, which barrier means may comprise spaced tubular elements 17having passages 18, 19 adjacent the second, bottom ends of the screens14, 15. Associated with each of the passages 18, 19 there preferably isprovided a baffle plate 20, which is mounted--as by horizontal tabs 21at opposite ends of the plate 20--between the passages 18, 19 and theinner faces of the screens 14, 15, to redirect the flow of backflushingliquid. Liquid which passes to the interior of the tube 17 ultimatelypasses through opening 22 in the upper bar 12 into the header 13, andultimately into the supporting arms for discharge from the diffuser.

As is illustrated by the arrows in FIG. 2 when the screen and pulp aremoving upwardly together--as illustrated on the left hand side in FIG.2--the spent treatment liquid flows through the screen 14, andultimately passes through the passage 18 into the interior of the tube17 and ultimately out the opening 22. When the screen assembly 10 ismoved rapidly downwardly--as illustrated in the right hand side of FIG.2--the liquid between the screens 14, 15 and the barrier means 16 flowsoutwardly through the screen. At the top, first end of the screen,adjacent the top bar 12, the backflushing liquid will essentially beliquid as clean as the liquid that was just withdrawn through the toppart of the screen at that point, meaning a slug of contaminated liquidwill not enter the pulp flow there. The dirtiest liquid will be at zone23, at the bottom of the screen; the pulp in zone 23 will have a chanceto be further treated with treatment liquid as it moves upwardly in thediffuser. The baffles 20 will redirect the flow of backflushing liquidat the bottom of the screen so that the backflushing liquid will nothave a tendency to merely flush out the bottom part of the screen butwill be redistributed relatively uniformly along the length of thescreens 14, 15.

One exemplary positioning of the screen assemblies 10 according to thepresent invention in an atmospheric diffuser is shown in FIG. 3. Theatmospheric diffuser is shown generally by reference numeral 30,including a generally upright vessel 31 having a plurality of radiallyextending arms 33 and with a pulp inlet 32 at the bottom of the vessel31. The arms 34 are connected to the headers 13 in conventional manners(as exemplified by said U.S. Pat. Nos. 3,815,386 and 4,172,036), thewithdrawn liquid passing through the arms 33 ultimately to a withdrawalconduit or connection 34, and being discharged into the discharge area35 outside of the vessel 31. The arms 33 are reciprocated up and down bythe hydraulic piston and cylinders 37 or like means. The arms 33--andassociated screen assemblies 10--are moved slowly upwardly in thedirection of pulp flow (at essentially the same speed as the pulp flow),but are moved rapidly downwardly (that is opposite the pulp flow) toeffect backflushing.

A central shaft 38 is preferably provided which rotates about a verticalaxis, and has a scraper 39 at the top thereof for the discharge oftreated pulp into the pulp discharge 40. Fluid treatment introductionstructures 42 are mounted to the shaft 38 for rotation therewith, anduniformly introduce treatment liquid--such as wash water or bleach--intothe pulp between the annular screen assemblies 10, 11. Note that in theexemplary construction in FIG. 3, the screen assemblies 10 according tothe present invention are provided just as the lower screen isassociated with the arms, whereas the upper screens 33 are indicated byreferenced numeral 1', and are comparable to the conventional screensillustrated in FIG. 1 (only they extend upwardly instead of downwardly).

FIG. 4 illustrates an arc of one of the screen assemblies 10 accordingto the invention, it being understood that the entire screen assemblywill be annular. Vertical supporting walls 44 are provided atpredetermined positions along the screen assembly 10, interconnectedbetween the upper and lower bars 12, 11, respectively, and between thetubes 17, webs 46 are provided. The webs 46 are desirable because toprovide tubes extending the entire circumference of the screen assembly10 would not be cost justified.

In the upper half of FIG. 4, a modification of the screen assembly 10,which modification is also according to the present invention, isprovided. In the modification illustrated in the upper part of FIG. 4structures comparable to those in the first embodiment are illustratedby the same reference, but only preceded by a "1". In this embodimentthe screen assembly 110 is substantially the same as the screen assembly10 except that the barriers are provided by channels 117 instead oftubes 17, with webs 146 extending between the channels 117.

The distinctions between the first and second embodiments will be moreclearly seen by inspection of FIGS. 5 and 6. Note that in the FIG. 6embodiment it is desirable to provided two openings 122 in the upper bar112, rather than one opening 22, associated with each of the channels117 since the channels 117 have a larger cross sectional area than thetubes 17. Note that the illustration in FIG. 5 is slightly differentthan that in FIG. 2. In FIG. 2, the passages 18, 19--one associated witheach of the screens 15, 14, respectively--are provided in each tube 17.In the FIG. 5 embodiment, each tube 17 has only one passage, thepassages 18, 19 alternating from one tube 17 to the next so that onepassage (18) is directed toward the inner surface of screen 15 for onetube, and then for the next tube 17 the passage 19 is directed towardthe inner surface of the other screen 14. The same alternatingconfiguration of the passages 118, 119 is illustrated in FIG. 6 for thechannel embodiment.

In the utilization of the atmospheric diffuser 30 in the treatment ofcellulosic fibrous material pulp in the upright vessel 31 using at leastone annular screen assembly 10, the following method is practiced. Thepulp is introduced into inlet 32 and flows vertically, upwardly, in thevessel 31, ultimately the treated (e.g. washed, thickened and/orbleached pulp) being discharged by scraper 39 into pulp outlet 40. Asthe pulp moves upwardly in the vessel 31, treatment liquid is introducedby conventional rotating liquid introducing nozzles 42, spent treatmentliquid passing through the screens 14, 15, ultimately flowing throughthe openings 22 into the headers 13 and arms 33, and being ultimatelydischarged into liquid outlet 35. This withdrawal action occurs duringthe entire upward movement of the screen assembly 10 by the hydrauliccylinders 37.

Once the screen assemblies 10 reach their upper limit of travel, thehydraulic cylinders 37 move the screen assemblies 10 downward rapidly.This causes the screens 14, 15 to be backflushed. Since the liquid thatwill be backflushing the upper portion (first ends) of the screens 14,15 is relatively clean liquid which has just been withdrawn through thatportion of screens, there will be no slug of relatively dirty liquidentering relatively clean pulp, as occurs in the prior art procedure ofFIG. 1. Thus, the backflushing is controlled--by the barrier 16 andassociated passages 18, 19 and located openings 22--so that efficiencyof the screen assemblies 10 is enhanced.

It will thus be seen that according to the present invention method,atmospheric diffuser, and screen assembly have been provided whichovercome a problem in the prior art that was identified as part of thepresent invention, providing for enhanced efficiency of the lowerscreens of a conventional diffuser. While the invention has been hereinshown and described in what is presently conceived to be the mostpractical and preferred embodiment, it will be apparent to those ofordinary skill in the art that many modifications may be made thereofwithin the scope of the invention, which scope is to be accorded thebroadest interpretation of the appended claims so as to encompass allequivalent apparatus, components, and methods.

What is claimed is:
 1. An atmospheric diffuser for treating cellulosicfibrous material pulp, comprising:a generally upright vessel; aplurality of arms mounted within said vessel for up and down movement; aplurality of annular screen assemblies mounted on said arms, eachassembly including at least one screen having holes therein; conduitmeans for transporting liquid passing into said screen assemblies frompulp in said vessel, into said arms and out of said vessel; at least onescreen of each screen assembly having an outer face and an inner face,said outer face first encountering the pulp as it moves in its directionof movement, with the arms operatively connected to the screen face atthe downstream end thereof in the direction of pulp movement; means formoving said arms up and down in movement coincident with pulp flow inone direction and to cause backflushing of liquid through the screens inthe other direction; means for directing backflushing liquid from saidinner face of said screen back through said screen so that thebackflushed liquid passing through the screen adjacent the downstreamend thereof has substantially the same properties as liquid justwithdrawn through the screen at that point wherein said arms are hollow,and further comprising means defining an opening from the inner face ofsaid screen to a hollow interior of said arms at a first end of saidscreen, said opening distinct from said screen holes; and wherein meansfor directing backflushing liquid comprises barrier means for preventingliquid which passes through said screen adjacent said first end thereoffrom directly passing into said opening, said barrier means extendingfrom said first end of said screen to adjacent a second end of saidscreen, remote from said first end, and means defining a passage in saidbarrier means adjacent said second end thereof.
 2. A diffuser as recitedin claim 1 further comprising a baffle means mounted adjacent saidpassage in said barrier means, said baffle means for redirecting theflow of backflushing liquid through said passage.
 3. A diffuser asrecited in claim 2 wherein said barrier means comprises a plurality ofhollow circular cross-section tubes, a passage formed in each tube.
 4. Adiffuser as recited in claim 2 wherein said barrier means comprises aplurality of channels, non-circular in cross-section a passage formed ineach channel.
 5. A diffuser as recited in claim 1 wherein at least oneof said screen assemblies comprises two concentric screens radiallyspaced from each other; and wherein said barrier means divides theannular space between said screens so that the inner faces of saidscreens are not in direct communication with each other.
 6. A diffuseras recited in claim 5 wherein said barrier means comprises a pluralityof tubes interconnected by webs, at least one passage defined in each ofsaid tubes.
 7. A diffuser as recited in claim 6 wherein two passages aredefined in each tube, each passage facing the inner surface of one ofsaid screens.
 8. A diffuser as recited in claim 6 wherein only onepassage is defined in each tube, the passage in one tube facing theinner surface of one screen, and the passage in the adjacent tube facingthe inner surface of the other screen, and this alternating sequence ofpassages being provided for all of the tubes.
 9. A diffuser as recitedin claim 6 further comprising a baffle means mounted adjacent saidpassage in said barrier means for redirecting the flow of backflushingliquid through said passage.
 10. A diffuser as recited in claim 5wherein said barrier means comprises a plurality of channels with websinterconnecting the channels, at least one passage being formed in eachchannel.
 11. A diffuser as recited in claim 5 further comprisingtreatment fluid introduction structures disposed between said annularscreen assemblies for introducing treatment fluid into the pulp flowingin the vessel.
 12. An assembly for processing pulp comprising:agenerally upright vessel; a plurality of arms mounted within said vesselfor up and down movement; at least one annular screen assembly includingat least one screen having a first end and a second end, and holesformed in said screen; means for connecting said screen assembly to saidarms adjacent said first end of said screen, said means including a walland means defining an opening for the passage of liquid therethrough insaid wall, said opening distinct from said screen holes; a barrierextending substantially the length of the screen, between the first andsecond ends thereof, and separating the screen from said opening in saidwall, and means defining a passage in said barrier adjacent the secondand of said screen, through which liquid must flow to enter said openingin said wall; and means for moving said arms up and down in movementcoincident with pulp flow in one direction, and to cause backflushing ofliquid through the screen in the other direction.
 13. Apparatus asrecited in claim 12 further comprising a baffle plate, and means formounting said baffle plate between said screen and said passage so thatsaid baffle plate redirects the flow of liquid passing from the side ofsaid barrier opposite said screen toward said screen.
 14. Apparatus asrecited in claim 12, wherein said screen assembly comprises twoconcentric screens, said barrier separating said concentric screens; andwherein said barrier comprises a plurality of tubes with webs extendingbetween the tubes, said passages formed in said tubes.
 15. Apparatus asrecited in claim 12 wherein said screen assembly comprises twoconcentric screens, said barrier separating said concentric screens; andwherein said barrier comprises a plurality of channels with websextending between the tubes, said passages formed in said channels.